1. Field of the Invention
The invention relates generally to drag-type drill bits and, more particularly, to the type of drag bit in which a plurality of cutters are mounted in a body.
2. Description of the Prior Art
Drag bits, of the type described, usually include a bit body having a cutting face with the cutters mounted thereon. The cutters usually comprise a carbide stud having an angled face for having a polycrystalline diamond (hereinmatter called PCD) compact affixed thereto, or a cylindrical piece of carbide having an end face for having the PCD compact mounted thereon. The stud cutters are usually brazed or force fitted into cylindrical holes formed in the cutting face, whereas the cylindrical cutters are usually placed on their sides into channels formed in the cutting face and are brazed thereto.
Conventionally there are two types of bit bodies utilized. One type is a steel body bit which normally had the cylindrical holes bored into the cutting face for receiving stud cutters. The other type is where the bit body is formed from a matrix material. The matrix body is formed in a mold and channels are normally formed on the cutting face to accept cylindrical cutters although initially stud cutters were also utilized on matrix bits. Normally cylindrical cutters were not used on steel body bits because the steel material would erode from around the cutters and the cylindrical cutters would fall out.
Early on in the development of these types of bits, the steel body bits had a relatively flat continuous cutting face, with the stud cutters extending outwardly therefrom. The original matrix bits also had similar face constructions.
Later on, bit bodies were formed with a plurality of blades extending downwardly to accept the cutters mounted thereon. Channels were formed between the blades to form fluid passages. The blades were either straight, radial blades or they were curved in a spiral fashion.
Initially the blades on steel body bits were formed with a bottom flat face that was parallel to the formation. The cylindrical cutters were mounted on the face with the cutting surfaces facing the direction of rotation. The problem with this construction is that the flat surfaces on the blade behind the cutting elements functioned as penetration limiters and also inhibited hydraulic flow passing by the cutting elements.
The matrix bits evolved to where matrix material was added to the surface of the blade directly behind the cylindrical cutters and along the sides thereof in order to add support to the cutting elements. These projections were easily formed on matrix bits by forming additional indentions of the mold surface forming the blades. These indentations accommodated the space for the cutters and for the additional supporting matrix material.
This type of construction can not be easily produced on a steel body bit, because machining such projections on the blade surface would be quite difficult. As a result steel body bits normally have not been able to utilize cylindrical cutters on the blades because of erosion problems and machining limitations.
U.S. patents illustrating the state of the art is given as follows: U.S. Pat. Nos. 4,073,354; 4,491,188; 4,558,753; 4,883 132; 4,898,252; 4,949,598; 4,995,887; 5,332,051; and 5,383,527.